Geochronologic Constraints On Neoproterozoic Major Events In Western Hubei And Their Geological Significances

The Neoproterozoic Era(1000-539 Ma)is a critical geological period having some significant environmental changes in Earth history,which witnessed important geological and biological events such as the breakup of the Rodinian supercontinent,"Snowball Earth" glaciations,the perturbation of marine carbon cycle,and the rapid radiation of metazoan.The evolutions in environment and life have reshaped Precambrian Earth planet.A reliable isotopic geochronologic framework is the basis for the studies of spatiotemporal processes and causal mechanism of the events above.In this study,we focus on some open questions rising from Cryogenic and Ediacaran periods in South China:including the duration of glaciation,time range and paleoclimate changes from glacial to interglacial,sediments provenance and paleogeography of Cryogenian System(i.e.,Nanhua System),and temporal sequence of Ediacaran paleoenvironmental and paleoclimatic events in South China.Here,we report five zircon U-Pb ages from volcanic tuffs in the Cryogenian Datangpo Formation and Ediacaran Doushantuo Formation in western Hubei,South China with LA-ICP-MS and high precision CA-ID-IRMS methods.According to the age data and geochemical proxies(e.g.,CIA and carbon and oxygen isotopes),we conclude main new results as below:1.The new LA-ICP-MS zircon U-Pb ages of the lower and upper tuff layers of the interglacial Datangpo Formation in Gucheng section of western Hubei are 660.1 ± 3.6Ma and 651.2 ± 3.3 Ma,and the CA-ID-IRMS zircon U-Pb ages are 660.33 ± 0.23 Ma and 651.64 ± 1.2 Ma,respectively.The new ages of 660.1 ± 3.6 and 660.33 ± 0.23 Ma are consistent with previously reported high-precision dates from the lower Datangpo Formation,South China.We infer that the Sturtian deglaciation occurred at ～661 Ma.The new age of 651.2 ± 3.3 Ma and 651.64 ± 1.2 Ma from the upper Datangpo Formation provides a maximum age constraint for the onset of the Marinoan glaciation in South China,which must have occurred after 654.5 Ma.In addition,the maximum duration for the Marinoan Ice Age is ～19 Myr.The new ages yield an estimated duration for the nonglacial interval between the Sturtian and Marinoan ice ages of ～9 Myr(maximum ～16Myr when age errors incorporated).2.The maximum depositional ages of the siltstones obtained from the lower and basal Datangpo Formation at Gucheng section are 659.0 ± 2.9 Ma and 661.6 ± 3.8 Ma,respectively.These ages further confirms that the deposition of the Datangpo Formation started at ～661 Ma.The detrital zircon chronology supports that the termination of the Sturtian glaciation in South China occurred at ～661 Ma.Based on comprehensive analysis of new ages and previously published data,we infer that the sediments in the Cryogenian Nanhua System in the southwestern Hubei Province were mainly derived from the northern and northwestern margins of the Yangtze Block.The pre-Neoproterozoic zircons were most likely derived from the Yudongzi,Kongling,Douling,Zhongxiang,Huangtuling complexes,whereas the Neoproterozoic zircons were mainly sourced from the Bikou Terrane,Hannan Massif,Wudangshan Group,and Huangling Batholith.In comparison with age distributions of detrital zircon from Australia,India,and Antarctica,the clastic sediments provenance of the Nanhua System in the southwestern Hubei Province was internally related with South China Craton.Thus,our results support the previous hypothesis that the South China Craton was located on the periphery of the Rodinia supercontinent rather than in its interior.3.The chemical index of alteration(CIA)and the petrological evidence of the Datangpo Formation at the Gucheng section in western Hubei suggest that chemical weathering of the Datangpo Formation was weak at early stage followed by a brief cold stage.There was a change in paleoclimate from cold to warm and humid before manganese-rich carbonate deposition of the Datangpo Formation.Another cold event occurred again before the sedimentation of the Nantuo Formation.The zircon U-Pb ages of the tuff in the lower and upper Datangpo Formation constrain two turning points in interglacial climate changes at ～660 Ma and ～651 Ma,respectively.4.The CA-ID-IRMS zircon U-Pb age of 551.6 ± 0.5 Ma has been obtained for the first time from the tuff layer at the top of the Miaohe Member of Doushantuo Formation in Qinglinkou section,western Hubei.Our chronostratigraphic and lithostratigraphic evidences,demonstrate the similarity of Miaohe-type Ediacaran stratigraphic sequence containing the Miaohe Member around the Huangling Anticline.This age is an important supplement to the age at the top of Miaohe Member in study area.In addition,new age confirms that stable carbon isotope negative excursion event(EN4)recorded in the Miaohe Member at Qinglinkou section is a carbon isotope negative excursion event that occurred again after the termination of the Shuram-equivalent negative excursion(EN3).The high-precision age of 551.6 ± 0.5 Ma at the top of Miaohe section of Doushantuo Formation suggest that the end of the negative excursion is close to ～550 Ma.